1. Field of the Invention
The present invention relates to a sound-transmitting membrane, and an electronic device equipped with the sound-transmitting membrane.
2. Description of Related Art
In recent years, electronic devices, such as mobile phones, laptop computers, electronic notebooks, digital cameras, and video-game instruments, generally have a sound function. A sound emitter such as a speaker and a buzzer, or a sound receiver such as a microphone, is disposed inside a housing of an electronic device having a sound function. An opening is provided in the housing of the electronic device at a position corresponding to the sound emitter or the sound receiver. Sounds are transmitted via the opening. In addition, in order to prevent foreign matters such as waterdrops from entering into the housing of the electronic device, a sound-transmitting membrane that allows passage of sounds and prevents passage of foreign matters is disposed so as to cover the opening.
As a sound-transmitting membrane, a porous plastic membrane is known which is produced by forming pores in a polytetrafluoroethylene (hereinafter, occasionally referred to as “PTFE”) film or an ultrahigh molecular polyethylene film (see JP 2003-53872 A).
JP 2003-53872 A proposes a sound-transmitting membrane in which a support is bonded to a porous plastic membrane, in view of ease of second processing of the sound-transmitting membrane, such as cutting, stamping, and bonding to a casing. Nets, nonwoven fabrics, and woven fabrics are mentioned as examples of the support. In addition, JP 2003-53872 A proposes a sound-transmitting membrane in which a support is bonded to a porous plastic membrane and of which the surface density is set within a predetermined range in order not to reduce the sound transmissibility of the sound-transmitting membrane.
JP 2004-83811 A proposes a water-proof sound-transmitting membrane that is a layered product composed of a plastic membrane and a support. Porous materials such as nets, foam rubbers, and sponge sheets, nonwoven fabrics, and woven fabrics, are mentioned as examples of the support.
In either JP 2003-53872 A or JP 2004-83811 A mentioned above, the acoustic characteristics of a sound-transmitting membrane using a nonwoven fabric as a support are not specifically discussed. In particular, the acoustic characteristics for a high-frequency range (for example, 3000 Hz) are not discussed at all.